The (c) on oreilly.com says, we are NOT allowed to redistribute the
document; whereas, the (c) on docbook.org says the book is under FDL,
wich
would allow me to modify and redistribute it, without the previous
permission of oreilly.

Which is the correct one? Can I redistribute the oreilly reference book,
or not? I don't really anderstand why there are two licenses... is this a
mistake?

IANAL, but I would think that the oreilly.com license page can not
revoke rights that the docbook.org license page clearly grants to you.